Zeolite crystal structures have found a wide range of applications within refinery processes and other processes for manipulating petroleum streams. Some zeolite applications are catalytic in nature, while other applications focus on the ability of zeolites to selectively adsorb molecules within a gas stream.
One example of a zeolite that is potentially suitable for selective adsorption of molecules is ZSM-58. ZSM-58 is a DDR-type zeolite with an 8-member ring structure. U.S. Pat. No. 4,698,217 describes a method for synthesizing ZSM-58 using a methyltropinium salt as the directing agent.
ZSM-58 is a relatively underdeveloped crystal, primarily due to the relatively high cost of the structure directing agent, a methyltropinium salt such as the iodide, and the low solids (e.g., low throughput) of the synthesis formulation. Historical work on this zeolite crystal has generally evaluated its ability to separate gas molecules based on the steric adsorption (i.e., molecular sieve effect) at equilibrium rate and capacity. The rate of adsorption of zeolites in general, and DDR/ZSM-58 in particular, was primarily dependent upon pore opening size. That is, the pore opening acted as a sieve to control entry into the zeolite and the adsorbent capacity of the zeolite.
However, when far from equilibrium rates (as in commercial-sized pressure swing adsorption units, or PSAs), kinetic effects can and often do lead to differences in component adsorption/separation, usually not towards better adsorption/separation but typically toward less effective adsorption/separation. In the past, ZSM-58 crystals have been plagued by slight defects (imperfections) that have the undesirable effect of allowing faster than desired transport through the zeolite crystal. Being able to remediate these defects and slow down the diffusion with a commercially viable process can be of paramount importance.
For CH4/CO2 separations, the diffusivity of CH4 typically determines the efficacy of the PSA unit being developed for commercialization. The efficiency of the separation is greatly dependent upon the diffusivity properties of the crystal. The slower the diffusion of CH4, the better the separation in the PSA.
Zero Length Chromatography (ZLC) can be used to measure diffusivities of gaseous components, particularly in difficult-to-engineer systems such as ZSM-58. The instant invention uses ZLC to track methane diffusivities, as synthesis and/or treatment parameters vary, allowing for quick validation of higher-desirability candidates for commercially viable processes and/or for further scientific research.